Skyrim's biggest city is little more than a brick walled Hla Oad.
I'd never thought about it that way. It's true, though.
I think part of the size of the cities comes down to the lack of different factions. I mean, in a town the size of the average Skyrim town, do you need more than on e tavern, more than one general store, etc.? Probably not. But if you have a bar where the Thieves Guild hang out, a bar where the Commona Tong hang out (or whatever the Nordic equivalent would be), a place for the Fighters and Mages, etc. ... suddenly you have a place that feels a lot bigger, even if essentially al you have are several different types of inn.
So for me, basically, more factions = better towns.